1. Field of the Invention
The present invention relates to an electro-deposition coated member. More particularly it relates to an electro-deposition coated member provided with an exterior coating comprising an electro-deposition coating film having electromagnetic wave shielding properties, that forms a housing used for optical instruments such as cameras, home electric appliances, office automation machinery, business machines, measuring devices or the like. It also relates to a process for producing such a member.
2. Related Background Art
In recent years, as electronic circuits have been made smaller size, increasingly complicated and precise, the misoperations and noise caused by electromagnetic waves generated from other component parts and circuits have presented significant problems. The electronic circuits themselves also generate electromagnetic waves, and also offer a significant problem by their influence on surroundings. In order to prevent these problems, it is sought to shield electronic circuits from invasion or radiation of electromagnetic waves.
As methods for shielding electromagnetic waves, a method is conventionally known in which a circuit substrate is surrounded with a metallic housing comprising a conductive material. However, as the products have been recently made small-sized and lightweight, it has been a prevailing practice to use a housing comprised of a plastic material. As a method of making such a plastic housing conductive, it has been a prevailing to use spray coating using a conductive coating composition. Other methods are also used which include zinc spray coating, electroless plating, vacuum deposition and conductive plastic coating.
The conventional methods, however, have the following disadvantages.
The conductive coating composition for spraying can achieve no sufficient electromagnetic wave shielding effect unless a conductive filler is contained in an amount of not less than 60 parts by weight and moreover a coating thickness is not less than 30 .mu.m in the case of a copper filler and not less than 50 .mu.m in the case of a nickel filler. For this reason, this coating composition is not suitable for decorative coating that provides an exterior coat on a housing.
In instances in which metal powder is used as the fillers, the metal powder has so large a specific gravity that it is required for the powder to be again dispersed when coating compositions are used, which, however, is not easy. To solve this problem, Japanese Patent Application Laid-open No. 59-223763 discloses a conductive coating composition for electromagnetic wave shielding in which Ni-coated mica powder is used as a conductive filler. This coating composition also can not achieve a sufficient electromagnetic wave shielding effect unless a coating is formed with a large thickness of 50 .mu.m or more.
In addition, in housings with complicated shapes, the coating thickness tends to be non-uniform, often resulting in an insufficient shielding effect.
As for the zinc spray coating, it must give a coating thickness of as large as from 50 to 100 .mu.m in order to ensure the shielding effect, and it also has a difficulty in adhesion to substrates. For this reason, it becomes necessary to provide steps for blast finishing, etc. In addition, there is still a problem in mass productivity because of a work environment worsened by zinc vapor gas.
In regard to the electroless plating, an electromagnetic wave shielding effect can be obtained when, for example, a copper coating is formed in a thickness of 1.0 .mu.m to 1.5 .mu.m or more. Since, however, the whole housing is plated, it becomes indispensable when used as a housing of a product, to form a coating film on the plated surface to improve the nice-looking appearance so that the commercial value can be enhanced. In doing so, however, there is a problem of the inadequacy in adhesion between the film surface formed by plating and the coating surface formed by coating. In particular, mere plating with copper may cause changes with time to bring about corrosion, resulting in a lowering of performances. Hence, the copper-plated surface must be subjected to nickel plating so that the quality can be prevented from being lowered. Moreover, since this nickel plating may greatly impair the adhesion to the coating film, the coating must be carried out using very limited materials such as special coating compositions as exemplified by Origiplate Z (available from Origin Electric Co., Ltd.). This greatly affect cost and can not be mass-productive.
On the other hand, a conductive plastic housing is known, which is formed of a mixture of a resin and a conductive filler such as metal powder with particle diameters of several tens .mu.m or metal fiber. The resulting plastic housing, however, has a too seriously uneven surface to be usable as an exterior member if it is used in the state of a molded product which is untreated or unfinished. Thus, there is the problem that decorative coating must be applied in order to attain commercial value. In addition, because of poor conductivity, any secondary finishing becomes necessary for achieving perfect electromagnetic wave shielding, which can not be mass-productive. Moreover, since conductive plastic materials themselves are expensive, there is also a limit on its practical utilization.
As for exterior coatings of housings, there is an increasing trend towards a high-quality appearance, accompanied with a high desire for a satin-like appearance. Accordingly, it is customary to carry out blasting before coating and thereafter carry out exterior coating. When, however, a housing with a shape having a terrace or a vertical angle is coated, a difference in coating thickness is produced between a plane portion and a terraced portion or vertically angular portion to cause defects such as lack of hiding, so that no high-quality satin-like appearance can be obtained.